IMO - B

'recovered' is parallel to 'founded' and 'known' is placed correctly in the sentence.

I picked A but the answer is indeed B.

Here is Tommy Wallach's(MGMAT Instructor) view on this question (Found this in some other forum).

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The fear of rabies is well founded; few people are known to recover from the disease after the appearance of the clinical symptoms.

(A)few people are known to recover from the disease after the appearance of the clinical symptoms.
PROBLEM: This question is a classic concision trap. The hope is that you'll pick A, thinking that A and B are the same and this one is shorter. But this is not idiomatic. We say "SUBJECT is known TO HAVE RECOVERED" not "SUBJECT is known TO RECOVER." We wouldn't say he is "known to eat twenty apples". The present tense infinitive (to eat) is wrong. Correct is that "I am known to have eaten 20 apples", which correctly uses the present perfect to imply this is what we've known from the past into the present.

Also, "after" and "once" have slightly different meanings. "I'm going to the store after lunch" implies that SOMETIME after lunch, I'll go to the store. "I'm going to the store once I've eaten lunch" implies that there is a causal relationship between the eating and the going to the store, that eating lunch is the threshold. That's what we want here, because the symptoms are the threshold.

(B)few people are known to have recovered from the disease once the clinical symptoms have appeared
ANSWER: Awesome.

(C)there are few known people who have recovered from the disease once the clinical symptoms have appeared
PROBLEM: Adjective "known" is now modifying "people", which is silly.

(D)after the clinical symptoms appear,there are few known people who have recovered from the disease
PROBLEM: Same as above, and the placement of the prep phrase "after..." is odd.

(E)recovery from the disease is known for only a few people after the clinical symptoms appear
PROBLEM: "Known for only a few people" doesn't make any sense. It's not that the recovery isn't known, but that few people are known to have recovered.

Hope that helps..

The fear of rabies is well founded; few people are known to recover from the disease after the appearance of the clinical symptoms.

(A) few people are known to recover from the disease after the appearance of the clinical symptoms -->Cream of the crap...What's the OA..

(B) few people are known to have recovered from the disease once the clinical symptoms have appeared ( the have appeared should be "had appeared AND also it should be "after the symptoms appeared"

(C) there are few known people who have recovered from the disease once the clinical symptoms have appeared ( few known people means that all those people are known..Meaning Issue
(D) after the clinical symptoms appear, there are few known people who have recovered from the disease( few known people means that all those people are known..Meaning Issue
(E) recovery from the disease is known for only a few people after the clinical symptoms appear Just plain awkward construction